In order to support the wheels such that they can rotate freely with respect to the suspension, various kinds of rolling-bearing units, which comprise an outer race and inner race that rotate freely by way of rolling elements, are used. Also, the rolling-bearing unit for supporting the driven wheels on the independent suspension is combined with a constant-velocity joint unit, and must be able to smoothly (maintaining constant velocity) transmit the rotation of the drive shaft to the aforementioned driven wheels, even when there is relative displacement between the differential gear and the driven wheels, or when a steering angle is applied to the wheels. A rolling-bearing unit, or so called fourth-generation hub unit, that can be combined with this kind of constant-velocity joint unit and which can be made to be relatively compact and lightweight has been known as disclosed in Japanese Patent Publication No. Toku Kai Hei 7-317754, Japanese Patent Publication No. Toku Kai Hei 10-264605, U.S. Pat. No. 5,674,011 and U.S. Pat. No. 5,853,250. None of these improved rolling-bearing units use a nut that would increase the weight, so it is possible to reduce the unspringing weight of the rolling-bearing unit for wheel support and make it more lightweight. Also, it is becomes more difficult for the reaction force from vibration of the automobile due to unevenness of the road to be transmitted to the vehicle body, and thus it is possible to improve the performance of the automobile such as riding comfort and driving stability.
In the case of the construction disclosed in Japanese Patent Publication No. Toku Kai Hei 7-317754, there is a hub having a pair of elements such that they are divided into an outside and inside in the axial direction and such that they are fitted and welded together. (The axially outside is toward the outside in the axial direction when the bearing unit is installed in the automobile, and is the left side in the drawings. Similarly, the axially inside is toward the center of the automobile and is the right side in the drawings.) In this first example of prior art construction, due to the heat during welding, it is possible for the shape of the inner-ring raceway portion that is formed around the outer peripheral surface of either one of the pair of elements to become deformed, or for the strength of this inner-ring raceway portion to decrease, and this it becomes impossible to sufficiently maintain durability of the rolling-bearing unit for wheel support that contains this inner-ring raceway.
In contrast, in the case of the construction disclosed in Japanese Patent Publication No. Toku Kai Hei 10-264605, U.S. Pat. No. 5,674,011 and U.S. Pat. No. 5,853,250, the rolling-bearing unit for wheel support and the constant-velocity joint are combined with each other using a spline joint such that rotation force can be freely transmitted. Also, the rolling-bearing unit for wheel support is positioned with respect to the constant-velocity joint in the axial direction by placing a connection member, which is elastically deformable in the radial direction, between and around the hub or inner race of the rolling-bearing unit for wheel support and the spline shaft section or housing section of the constant-velocity joint. (They are connected such that they cannot be separated.) In the case of this second example of prior art construction, there is no need to weld together the component members of the rolling-bearing unit for wheel support, so it is possible to adequately maintain the durability of the rolling-bearing unit for wheel support that contains the inner-ring raceway. Moreover, the connection between the rolling-bearing unit for wheel support and the constant-velocity joint is accomplished using a connection member other than the nut, it is possible to simplify the work of combining the rolling-bearing unit for wheel support and the constant-velocity joint to make a wheel drive unit, and thus it is also possible to reduce the manufacturing cost of the wheel drive unit.
In the case of the second example of prior art construction described above, it was possible to make the wheel drive unit comprising a rolling-bearing unit for wheel support combined with a constant-velocity joint more compact and lightweight, however, in order to make the wheel drive unit even more lightweight while at the same time maintain its reliability, it is desirable that improvements be made to the area of maintaining a minimum of functions even when the connection member is damaged. In other words, in order to further lighten the weight, it is desirable that the cross-sectional area of the connection member be decreased in order to make the aforementioned connection member more compact for the more light-weight unit, however, in that case, it is necessary to consider what will happen if for some reason the connection member breaks.
When the cross-sectional area of the connection member is made small, it becomes impossible to say whether or not the function of the connection member for positioning the rolling-bearing unit for wheel support and the constant-velocity joint would be lost if the connection member were to break due to corrosion or cracking. Moreover, in the case that the rolling-bearing unit becomes completely separated from the constant-velocity joint due to loss of this positioning function, there is a possibility that the components of the constant-velocity joint could fall out.